1. Field of the Invention
The present invention relates to a magnetic recording medium and a magnetic recording device.
2. Description of the Related Art
In recent years, an amount of information processed by a computer or the like is increasing at a significant rate, and recording equipment used with the computer is required to attain higher recording density. Among many recording media, magnetic recording media such as magnetic disks are historically older than the other media and widely used.
Most of magnetic recording media supplied to the market to date are in-plane magnetic recording media in which direction of magnetization recorded in a recording layer is directed in the in-plane direction. To obtain higher recording density in the in-plane recording media, for example, thickness of the recording layer is reduced, and size of magnetic crystal grains constituting the recording layer is reduced for reduction in interaction between the magnetic crystal grains. However, the magnetic crystal grains reduced in size like this cause decrease in thermal stability and cause a phenomenon that information is lost by heat applied to the magnetic disk. Such a phenomenon is called thermal fluctuation and contributes to preventing higher recording density.
As a magnetic recording medium which achieves higher recording density without reducing the size of magnetic crystal grains, therefore, a perpendicular magnetic recording medium has attracted attention in recent years, in which the direction of magnetization in the recording layer is directed to a perpendicular direction to the in-plane direction of the recording layer.
According to the perpendicular magnetic recording medium, compared to the in-plane magnetic recording medium, each magnetic domain requires a smaller area in the surface of the recording layer, and higher recording density can be achieved. Furthermore, the magnetization is directed to the perpendicular direction to the in-plane direction of the recording layer, and accordingly, the recording layer can be made thicker. The thermal fluctuation, which is caused in a thin recording layer, is therefore less likely to occur.
As a recording layer of the perpendicular magnetic recording medium, a granular recording layer is attracting attention recently. The granular recording layer includes columnar magnetic crystal grains long in the perpendicular direction of the recording layer, and the columnar magnetic crystal grains are separated from each other by an oxide or a nitride. For, example, a CoPt alloy or the like is used for the magnetic crystal grains.
In the granular recording layer, when the content of the oxide or nitride is increased, the perpendicular magnetic anisotropy increases excessively. In this case, it is difficult to reverse magnetization of the granular recording layer using a magnetic head, and writing information to the granular recording layer becomes difficult.
On the other hand, if the content of oxide or nitride is reduced, the magnetic anisotropy of the granular recording layer decreases, thus facilitating writing information to the granular recording layer. However, the direction of the magnetization of the magnetic crystal grains is easily reversed by heat, and the aforementioned heat fluctuation occurs.
As described above, in the perpendicular magnetic recording medium using the granular recording layer, there is a trade-off relationship between the writing performance and the resistance to thermal fluctuation. To achieve a high recording density of, for example, 200 Gbit/in2 or more in the future, how to strike a balance between these properties becomes an issue.
As a technology related to this application, Patent Literature 1 discloses that the recording layer is composed of a plurality of granular magnetic layers having different oxide contents and the oxide content of the lowermost granular magnetic layer is the highest in the recording layer.
Patent Literature 2 discloses that as a soft magnetic layer formed under the recording layer, a soft magnetic layer having perpendicular magnetic anisotropy and a soft magnetic layer having in-plane magnetic anisotropy are formed in this order.
The technologies related to the present invention are also disclosed in the following Patent Literatures 3 and 4.    (Patent Literature 1) Japanese Patent Laid-Open publication No. 2004-259423    (Patent Literature 2) Japanese Patent Laid-Open publication No. 2004-227666    (Patent Literature 3) Japanese Patent Laid-Open publication No. 2001-148109    (Patent Literature 4) Japanese Patent Laid-Open publication No. 2005-044415